File and directory permissions can be changed with the chmod command (which accepts numeric and symbolic values).

The owner of a group can be changed with the chown command whereas the chgrp command allows changing he group associated with a file.

The du command can show how much of a disk is used.

The df command shows how much of a disk is free.

The main tool for troubleshooting disk issues is fsck which can check file-system structure, including inodes.

To mount file-systems, use the mount command and to unmount them use umount. To have mounting occur automatically at startup, add the entries to /etc/fstab.

Quotas can restrict the amount of space users or groups can use on the disk. Quotas are initialized with the quota command and they are toggled on and off with quotaon and quotaoff. They can be changed with edquota and reports can be generated with repquota.

When files are first created, the default permissions are equal to 666 minus any umask values. The default permissions for directories is equal to 777 minus any umask values.